Phys. Educ 25 (19531 Printed 4" Ihe UK
Everyday risks
George Marx
This paper, presented at the 1991 Pan-American
Science conference In Venezuela, was one of the
keynote addresses and was warmly received by
the delegates.
The author, George Marx, who Is Professor of
Atomlc Physics at Eiihr6s University In Budapest,
is very well known In his own country but also has
an internationalreputation.
George M a n is President of GIREP (Groupe
International de Recherche sur I'Enselgnement
de la Physique) and organizes and contributes to
many International conferences In Hungary and
around the world. George IS also VIcaChalrman
of the IUPAP Commission on Physics Education
and a member of the InternationalAdvisory Panel
for Physics Educaffon.
The first part of the paper, entitled 'Everyday
risks', appears in this Issue of Phydcs E d w f f o n
and will be followed by lhe second part ('Risks of
radloactlvky') In the March Issue and the flnal part
('Publlc risks due lo nuclear industry') in the May
Issue.
John U Avlson, Honorary Editor
'When you cross a road, look to the right at first,
then look to the left at the middle of the road'-we
used to tell our children and pupils, but we don't
add:-'look up as well, to see whether a chimney is
tumbling down or whether an aeroplane is falling
on your head'. The latter events still han a 6nite
probability! If two people die of these accidents in
one year in the United States, the probability that a
citizenwilldieis 1/100OWOM)years,i.e.lO-*/year.
By everyday experience, common sense judges
such a risk negligible @racticallyzero).
The mathematical dehition of risk is R = PC,
where P is the probability of occurrence and C i s
the seriousness of the consequence. (In the case of
certainty, P = l . In the case of death, C=l).
According to the definition of probability, if N
people are exposed to the same risk R, the collec~31-~120/531DlW22
t Cd $O75O@ 1$93IOP PobhshingU d
five risk (i.e. the expected number of lethal casualties due to this exposure) is NR. For a simple
discussion, let us introduce the concept of microi.e. as a risk that may kill
risk as I/million
one from among one million people exposed.
According to international assessment, one microrisk is incurred when
travelling 2500 km by train,
flying 2000 km by plane,
travelling 80 km by bus,
driving a car for 65 km,
bicycling for 12 km,
riding a motorcycle for 3 km,
smoking 1.5 cigarettes,
living two months with a smoker,
'
drinking halfa litre ofwine,
living in a brick house for ten days,
breathing in a polluted city like Budapest for
three days.
Looking at these numbers, one may conclude that
people consider a few microrisks affordable:
1 microrisk is about smoking a cigarette, or driving
your car to the next town or riding a motorcycle to
pick up a friend. As a matter of fact, in legal terms
the Congress of the USA considers one microrisk
to be negligible.
The 'right of knowledge' act (accepted by the
Stateof California with a majority of two-thirds in
1987) states that 'nobody may be exposed-consciously or unconsciously-to a chemical effect
that may cause cancer or genetic harm, without
calling the attention of the person to be exposed to
this danger.' But in court one must know: what
does a punishable non-zero risk mean? A physicist
may be inclined to say: 'What I can measure'.
According to the legal praxis in California, an
exposure above IO microrisks must not be caused
without advanced warning. This is why health
packet of
warnings
. must be printed on every
~.
cigarettes.
One microrisk may look small in itself. But let
us consider a state of N = IO million inhabitants
Oike Hunwary). If each person is exvosed to the
'affordable' i 'microrisk; this means- a collective
risk N R = IO. Ten innocent casualties does not
look such a low price any longer! This example
shows that the presentation of risk offers a chance
to manipulate the public. For example, after the
Three Mile Island nuclear accident a local newspaper wrote: ‘The emission of active noble gases
increased the risk of a person living in that environment by the equivalent of smoking half a cigarette.’
(It is reassuring, isn’t it?) The three million people
living in the affected environment were informed
by another local newspaper that: ‘The irresponsibility of technocrats kills two innocent victims!’
(It is terrible, isn’t it?) Simple multiplication shows
that the two statements are equivalent! Anyone
who quotes numerical data to the public or to
students has to do it with the utmost responsibility.
Society can be educated for responsible democratic
decision-making (e.g. about the route of progress)
by being schooled in rational thinking and by
obtaining relevant information.
Chemical risks
Alvin Weinberg recalled recently that emotional
anxiety may be felt in society about the dangers of
modem science and technology. This is why he has
named the (otherwise peaceful) 1980s ‘the age of
anxiety’. One component of anxiety is a lack of
scientific/technical knowledge about low-level
risks. (Even more ignorance may be experienced
by citizens, journalists, and in some countries even
by the decision-makers.)
Yes, it is a hard fact of life that each of us has to
die-sooner or later. (By being born each of us
takes a risk R = 1 to die.) But in the 20th century
life expectancy has increased from 35 years to
70 years in Hungary! It is true that the number of
victims of tuberculosis went down from 18 000 to
1000 per year in the same period, but deaths from
lung cancer rose from 350 to 6400 per year. In
Hungary, air pollution is estimated to be the cause
of 6% deaths, which means IO 000 casualties per
year, comparable to the number of victims of
smoking).
As the saccharin controversy-among othersindicates, it is rather difficult to make a quantitative assessment in the case of chemical risks. For
simplicity, the Environmental Protection Agency
(USA) has adopted a simple proportionality
between dose and risk (with no threshold). For
example the slope of this straight line is 100
microrisk/g of arsenic. In Hungary, the maximum
allowed arsenic content of drinking water from
country wells is 0.05 mgllitre. This means that
drinking half a litre of water per day from such a
well gives you a microrisk of cancer in a year.
(There are some country wells with an arsenic
concentration over ten times higher. In the last
decade, drinking their water has been forbidden
because the risk of drinking it exceeds IO microriskslyear.)
LetfheSun shine?
‘If you don’t go out in thegunshine, you may get
rachitis (rickets)’-we were told by grandpa. It is
true: the near infrared radiation contributes to our
production ofvitamin D.
The first humans emerged in Africa; they were
evidently dark-skinned. When some of them were
driven by overpopulation to cloudy Europe, a
mutation decreasing the pigment production was
an advantage: the skin collected more sunshine, so
the body could produce more vitamin D.This is
why doctors recommend a sun-lamp for the long
dark winter afternoons in Northem Europe.
The hard ultraviolet photons of sunshine break
up the molecules of air, which is how the ionosphere has been produced. Deeper atmospheric
layers are reached only by soft ultraviolet photons
(0.5-0.7 aJ) and by visible photons (0.25-0.5a n .
In the first billion years after the creation of
the Earth the bombardment of soft ultraviolet
photons made the survival of complex organic
molecules impossible; life could not evolve on
land. The green plankton in the sea, however,
began to pump oxygen into the atmosphere by
photosynthesis (bv+C02 + C+Oz), and the
ultraviolet photons broke up the oxygen molecules,
producing ozone (bv+Ol O+O,0 2 + 0+ 03).
The valence angle of ozone (0,)
is about 120”, it
contains delocalized electrons on a pathway
0.25 nm long. These absorb the near ultraviolet
photons (bv= 0.6 aJ). Under the protection of this
ozone shield lifedared to occupy thecontinents.
In the 1980s British scientists noted that at
springtime the thickness of the ozone shield
dropped to one-sixth of its usual value above
Antarctica. The ozone hole reached a record size
in 1990. The suspects were found on the spot: they
were Freon-type molecules (CFCs), used in sprays,
in refrigerators and in air conditioners. These
man-made molecules are durable enough to diffuse
up to the stratosphere, where they catalyse the
decay of ozone. Ultraviolet photons cross the
broken ozone shield, they harm leaves and may
cause skin cancer in human beings.
The populations of the USA and Australia are
especially sensitive (think of President Reagan’s
nose): they are pale skinned people who like to get
a sun-tan. Skin cancer is three times more common
in sunny Texas than in rainy Iowa. Most skin
cancers occur in Australia. The number of skin
cancer cases has doubled in 20 years and quadrupled in 40 years even in Europe.
-f
23
According to the Environmental Protection
Agency (USA) 1 % thinning of the ozone layer may
increase the ultraviolet radiation by 2%. This
could cause 6000 extra megamelanoma cases in the
USA and several tens of thousands worldwide.
The most recent measurements indicate that the
decay of the ozone layer has already exceeded this
value. This is why a sun-tan is already out of
fashion in California and on the Riviera. This is
why the Montreal Protocol urges a suppression in
the use of Freon-type compounds.
Ultraviolet radiation is harmful because it
excites and destroys organicmolecules.
We shall focus our attention on ionizing radiation, not only because radioactivity is the most
feared, but because it can be measured, checked,
researchedandcontrolled themost easily.
Elettron
U
Radioactivity
The unit of activity of a sample is 1 Bq (becquerel)
= I decay/second. (If a sample contains N radioactive nuclei of half-life T,then its activity can be
calculated as A = 0.7N/T.)
Radioactive decay liberates energy: it produces
ionizing radiation, This radiation may destroy
molecules in the human body, disturbing the delicate network of the biochemical metabolism. The
overall number of ions may be considered to be a
measure of the effect of this radiation. The dqse is
the ratio of the absorbed ionization energy E to
body mass M, that is E/M.
(The corresponding unit is 1 Gy = 1 gray= I
joule/kg= I J/kg. The differences in the biological
effects of different particles can be taken into
account by a quality factor Q. The electrons and
x-ray quanta produce ions with a lower probability than u-particles (see figure I), so they have
a smaller chance to overload a single cell with ions;
the defence system of the cell can cope with them
more easily: Q = I. The harm may be greater for
heavier particles (Q > I). The dose equivalent is
defined as D = QE/M. Heavier charged particles
are absorbed easily; therefore the public are
exposed mostly to x-rays, gamma-rays and e l m
trons. For the understanding of everyday risks this
distinction is not so relevant.)
The unit of dose equivalent D is I Sv (sievert)=
I J/kg (for x-ray, beta and gamma radiation). We
know from the bitter experiences of Hiroshima
and Nagasaki that D > IO Sv is lethal. D = 4 Sv
results in death with a probability of 50%. A dose
of a few Sv causa acute symptoms (loss of hair,
bleeding in the gut) within days. In everyday life
much smaller doses occur, and therefore we shall
use 1000 times smaller units: 1 mSv(millisievert)=
1 SV/1000.
24
Alpha particles create more ions in a single
cell than other particles.
Figure 1.
There was a zone in Hiroshima and Nagasaki (a
belt at a distance of 1.5-2.5 km around the epicentre) where people survived but received radiation doses of about 100mSv. (Experts have
reconstructed the dose for each of these people:
where were they? indoors or outdoors? what was
the roof constructed of? etc.) Their medical history
and the causes of death were tracked carefully. The
statistics obtained have been compared with those
of the Japanese population living elsewhere. The
estimation obtained by subtracting the normal
mortality and by extrapolation, assuming a linear
proportionality between risk and dose, has shown
that a dose equivalent of I mSv iccreases the risk,
of lethal leukaemia and cancer by about 50 microrisks. (A similar medical follow-up is going on in
Chernobyl.) The International Commission on
Radiological Protection recommends this risk/
dose factor in official calculations. (At much
higher doses the factor is taken to be twice as large,
but such high doses do not affect the public.) So
what is the risk of I mSv dose equivalent?SO lethal cancer cases per million people exposed.
Equally risky are
to smoke four packets of cigarettes,
to bicycle for 600 km,
to drive for 3250 km,
to cross a busy road twice a day for a year,
to be x-rayed for kidney metabolism.
Are you prepared to take such a risk? (The
expected answer is: ‘it depends, for what?)
(The energy of a gamma-photon of natural
radioactivity or a medical x-ray may be about
J. 1 mSv=lO-’J/kgresults
0.1 MeV=1.6x
in 50 microrisks. But the impact of a single
photon-at a critical site-may cause cancer! This
means that the attack of a single gamma-photon
on a body of 75 kg creates cancer with a probability50x 10-6x 1.6x 10-’2/75=10-’s--aninconceivably small number! But one must not forget
the definition of risk a roulette ball is rolling,
there are 10” numbers written on the dish, but at
stake is IiJe or death. A single photon may cause a
lethal disease, like a single arsenic atom, a single
benzene molecule or a single AIDS virus. Therefore
international regulation requires that the radioactivity suffered by people has to be as low as
reasonably achievable: the ALARA principle. Translated, e.g., to the praxis of x-ray lung screening, it
requires that the number of curable cases of lung
cancer detected must be larger than the number of
leukaemia cases caused by the x-raying.)
The law says that the artificial radiation burden
on the population must not exceed 5 mSv/year
(corresponding essentially to 5 microrisks/week).
Medical interventions to save life may and do
surpass this value. For those who work professionally with radiation the dose limit is 50 mSv/year.
(the largest exposure within the Hungarian Nuclear
Power Station was 33 mSv in a year.)
As we understand cancer, it begins when a cell
becomes antisocial. The cell starts to multiply,
spreading and wandering to any part of the body.
Mice and rats have ten thousand times fewer cells
and their lifetime is a hundred times shorter. It
follows that theoretically their abundance of cancer should be a hundred thousand times lower
than in human beings. But experience shows that
they get cancer as frequently as people! By growing
large, we have learned to protect ourselves against
chemical and ionizing attacks pretty well.
Why can radiation be harmful?
We are made mostly of water. The most probable process of ion formation is
photon+H,O+H++OHOH+OH
+H20z
(ion!)
(oxidant!)
The charged ions and hydrogen peroxide molecules both disturb the biochemical network, finely
tuned by enzymes within the reducing envuonment of a cell. A similar harmful attack happened
when the photosynthesis of plants enhanced the
oxygen in the atmosphere. The land animals
developed an efficient defence against the attack
of active oxygen (peroxide): catalase and superoxide-dismutase enzymes. This means that
oxygen-breathing and ionizing radiation attack
the cell metabolism in a similar way. The cells may
delay these harmful consequences, but there is no
complete defence. If 1 mSv/year dose equivalent
means 50 microriskslyear, a person at the age of
60 years has collected a cancer risk of 0.3% due to
this radiation dose. But 20% of people will die
anyway of cancer! Therefore James Lovelock
(the initiator of the Gaia hypothesis) argues that
breathing air is equivalent to a radiation dose of
66 mSv per year. Should we stop breathing? (Silly
question.)
The chemical affinity of atmospheric oxygen has
made intense biological activity possible: it has
created animals, human beings, cultures. Our
bodies have developed a rather reliable defence
against its dangers. That is why we can live so
long. This defence works against ionizing radiation as well. A gamma-photon will harm us with
the very low probability R = IO-”.
On the basis of these numbers we shall discuss
the risks of radioactivity affecting the population
in the March issue of the journal,